1. Field of the Invention
The present invention relates to a lithographic printing plate precursor and a lithographic printing method for using the same. Specifically, the present invention relates to a lithographic printing plate precursor capable of so-called direct plate-making, i.e., capable of directly making a printing plate by scanning a lithographic printing plate precursor with laser beams on the basis of digital signals of, e.g., a computer, and a lithographic printing method of developing the lithographic printing plate precursor on a printing press and performing printing.
2. Description of the Related Art
A lithographic printing plate generally comprises a lipophilic image area that receives ink and a hydrophilic non-image area that receives a fountain solution in printing. Lithographic printing is a printing method of making difference in ink-adhering property on the surface of a lithographic printing plate with the lipophilic image area of the lithographic printing plate as the ink-receptive area and the hydrophilic non-image area as the fountain solution-receptive area (ink-repellent area) by making use of the natures of water and oily ink of repelling to each other, adhering ink only on the image area, and transferring the ink to the material to be printed, e.g., paper.
For manufacturing this lithographic printing plate, a lithographic printing plate precursor (a PS plate) comprising a hydrophilic support having provided thereon a lipophilic photosensitive resin layer (an image-recording layer) has so far been widely used. The lithographic printing plate is generally obtained by a plate-making method of exposing a lithographic printing plate precursor through an original image of a lith film and the like, and then, for leaving the area to become an image area of the image-recording layer, dissolving and removing other unnecessary image-recording layer with an alkali developing solution or an organic solvent, to thereby bare a hydrophilic support surface to form a non-image area.
In a related-art plate-making process of a lithographic printing plate precursor, a process of dissolving and removing unnecessary image-recording layer with a developing solution and the like after exposure is necessary, but the exclusion or simplification of such an additional wet process is one of the objects in the industry. Since the disposal of waste solutions discharged with wet processes is a particularly great interest in the industry at large in recent years from the consideration of the global environment, the solution of the above problem is increasingly desired.
As a simple plate-making method, a method that is called on-press development is proposed, which is a method of using an image-recording layer capable of removing an unnecessary area of a lithographic printing plate precursor in an ordinary printing process, and removing a non-image area after exposure on a printing press to obtain a lithographic printing plate.
As the specific examples of on-press development, e.g., a method of using a lithographic printing plate precursor having an image-recording layer soluble or dispersible with, e.g., a fountain solution, an ink solvent, or an emulsified product of a fountain solution and ink, a method of mechanically removing an image-recording layer by the contact with the rollers and the blanket of a press, and a method of mechanically removing an image-recording layer by the contact with the rollers and the blanket after weakening the cohesive strength of an image-recording layer or the adhesive strength of an image-recording layer and a support by the permeation of a fountain solution and an ink solvent are exemplified.
In the present invention, unless otherwise indicated, “development process” means a process of removing an unexposed area of an image-recording layer of a lithographic printing plate precursor by being brought into contact with a liquid (generally an alkali developing solution) to thereby bare the hydrophilic support surface with an apparatus other than a printing press (generally an automatic processor), and “on-press development” means a method and a process of removing an unexposed area of an image-recording layer of a lithographic printing plate precursor by being brought into contact with a liquid (generally printing ink and/or a fountain solution) to thereby bare the hydrophilic support surface with a printing press.
On the other hand, in recent years, digitized techniques of electronically processing, accumulating and outputting image data using a computer have prevailed, and various image output systems corresponding to these digitized techniques have been put to practical use. Under such circumstances, a computer-to-plate technique directly making a printing plate is attracting public attention, which comprises scanning exposing a lithographic printing plate precursor with high convergent radiant rays such as laser beams carrying digitized image data without using a lith film. With such a tendency, it is an important technical subject to obtain a lithographic printing plate precursor well adapted to this purpose.
Accordingly, in recent years, the simplification of plate-making operation, and the realization of dry system and non-processing system have been further strongly required from both aspects of the above-described global environmental protection and the adaptation for digitization.
As such a lithographic printing plate precursor, e.g., a lithographic printing plate precursor comprising a hydrophilic support having provided thereon an image-forming layer containing a hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder is known (refer to, e.g., Japanese Patent 2938397). After being subjected to exposure with an infrared laser and the hydrophobic thermoplastic polymer particles are fused and coalesced by heat and an image is formed, the lithographic printing plate precursor is attached to the cylinder of a printing press and subjected to on-press development by feeding water and/or a fountain solution.
However, such a method of forming an image by coalescence due to mere heat fusion of polymer particles certainly shows good on-press developing property, but image strength is extremely weak and press life is insufficient.
As the example of improving the press life of a lithographic printing plate precursor capable of on-press development, a lithographic printing plate precursor comprising a hydrophilic support having provided thereon a heat-sensitive layer containing microcapsules containing a compound having a heat reactive functional group, wherein the heat-sensitive layer or a contiguous layer to the heat-sensitive layer contains an infrared absorber, is suggested (refer to JP-A-2001-277740 and JP-A-2001-277742).
Further, as another technique for improving the press life, a lithographic printing plate precursor capable of on-press development which comprises a support having provided thereon a photosensitive layer containing an infrared absorber, a radical polymerization initiator and a polymerizable compound is known (refer to JP-A-2002-287334). Further, JP-A-2000-39711 discloses a lithographic printing plate precursor comprising an aluminum support and using on the aluminum support (a) a water-soluble or water-dispersible polymer, (b) a monomer or oligomer having a photopolymerizable ethylenic unsaturated double bond, and (c) a photosensitive composition containing a photopolymerization initiator having absorption maximum in the ultraviolet region, which lithographic printing plate precursor is capable of on-press development after exposure.
In these methods of using a reaction, e.g., a polymerization reaction, the density of chemical bonding of the image area is high as compared with the image area formed by the heat fusion of polymer fine particles, therefore it is possible to improve image strength. However, on-press development is not sufficiently compatible with fine line reproducibility and press life.
On the other hand, in an on-press development type lithographic printing plate, an overcoat layer (a protective layer) is generally provided on an image-recording layer for the purpose of preventing the generation of scratches, shielding oxygen, and preventing ablation by the exposure with high illuminance laser beams. Water-soluble polymer compounds excellent in crystallizability, e.g., polyvinyl alcohol, are used as the main component in the overcoat layer, but further improvements of on-press developability, fine line reproducibility, press life, and on-press development running properties (aptitudes, e.g., the mixing of the substances removed by on-press development in a fountain solution, and the accumulation of the substance on an impression cylinder in continuously repeating on-press development and printing) are required.
To cope with this problem, JP-A-11-38633 contrives the increase of sensitivity and the storage stability of a photosensitive material by using an inorganic laminar compound in an overcoat layer, but the photosensitive material disclosed in this patent is a negative image-recording material by alkali development, which is different from the material of the type for use in on-press development.